Spark plug

Abstract

A sparkplug includes a ground electrode forming a gap with a front end surface of the center electrode. A front end portion of the ground electrode includes an opposed surface facing the center electrode, and a pair of tapered surfaces sandwiching the opposed surface. A shortest distance between the center electrode and a boundary formed by the opposed surface and the tapered surface is equal to or less than 1.2 times a distance of the gap. At least a part of a cross section of the core portion is disposed in a region at a front side of the straight line that passes a rear end of a line segment corresponding to the tapered surface and is vertical to the line segment. A shortest distance between the line segment and the cross section of the core portion is 0.2 mm or more and 1.5 mm or less.

What is claimed is: 
     
       1. A sparkplug, comprising:
 a center electrode extending in an axial direction; 
 an insulator with an axial hole extending in the axial direction, the center electrode being disposed to be inserted into the axial hole; 
 a metal shell disposed at an outer circumference of the insulator; and 
 a ground electrode that electrically connects to the metal shell, the ground electrode forming a gap with a front end surface of the center electrode, wherein 
 the ground electrode includes a rod-shaped main body portion, the main body portion including a base material and a core portion, the base material forming at least a part of a surface of the ground electrode, the core portion being buried in the base material and having a higher thermal conductivity than the base material, 
 a front end portion of the main body portion of the ground electrode is disposed at a position facing a front end surface of the center electrode, 
 the front end portion of the main body portion includes a tapered front end portion, the tapered front end portion including an opposed surface and a pair of tapered surfaces, the opposed surface being a surface facing the center electrode, the tapered surfaces being configured to sandwich the opposed surface, 
 when the front end surface of the center electrode is projected along the axial direction, at least a part of the tapered end portion is disposed in a range overlapping the projected front end surface of the center electrode, 
 a shortest distance between the center electrode and a boundary formed by the opposed surface and the tapered surface is equal to or less than 1.2 times a distance of the gap, and 
 a perpendicular cross section of the ground electrode includes a front end of the core portion perpendicular to the axial direction, wherein 
 at least a part of a cross section of the core portion is disposed in a region at a front end side of a straight line that passes a rear end of a line segment corresponding to the tapered surface and is vertical to the line segment, and 
 a shortest distance between the line segment and the cross section of the core portion is 0.2 mm or more to 1.5 mm or less. 
 
     
     
       2. The spark plug according to  claim 1 , wherein
 at least a part including the front end of the core portion is formed of a material with a melting point of 1350 degrees Celsius or more. 
 
     
     
       3. The spark plug according to  claim 2 , wherein
 the core portion includes a first core portion and a second core portion, the first core portion having a higher thermal conductivity than the base material, the second core portion being disposed between the base material and the first core portion and having a higher thermal conductivity than the first core portion, 
 on the perpendicular cross section, a cross-sectional structure of a front end side of the ground electrode is a two-layered structure of the first core portion and the base material, a cross-sectional structure of a rear end side of the ground electrode being a three-layered structure of the first core portion, the second core portion, and the base material. 
 
     
     
       4. The spark plug according to  claim 3 , wherein
 the ground electrode further includes a noble metal tip, the noble metal tip facing a front end surface of the center electrode. 
 
     
     
       5. The spark plug according to  claim 2 , wherein
 the ground electrode further includes a noble metal tip, the noble metal tip facing a front end surface of the center electrode. 
 
     
     
       6. The spark plug according to  claim 1 , wherein
 the ground electrode further includes a noble metal tip, the noble metal tip facing a front end surface of the center electrode. 
 
     
     
       7. The spark plug according to  claim 6 , wherein
 the noble metal tip is secured to the base material, 
 the main body portion of the ground electrode includes a fusion portion, the fusion portion including a constituent of the base material and a constituent of the noble metal tip, 
 a dividing cross section that is perpendicular to the opposed surface has a line that uniformly divides the opposed surface into two, the line extending on the opposed surface in a cross section of the ground electrode along a longitudinal direction of the ground electrode, wherein 
 among straight lines perpendicular to a direction where the opposed surface extends and overlap a cross section of the fusion portion, a straight line at a most front end side is referred to as a first straight line and a straight line at a rearmost end side is referred to as a second straight line, 
 an area of the cross section of the fusion portion is referred to as a first area S 1 , 
 on a cross section of the main body portion of the ground electrode, an area of a part sandwiched between the first straight line and the second straight line is referred to as a second area S 2 , 
 an area ratio S 1 /S 2  is less than 1/3, a cross section of the core portion extends to a front end side of the ground electrode with respect to the second straight line, and the cross section of the core portion is away from a cross section of the fusion portion. 
 
     
     
       8. The spark plug according to  claim 6 , wherein
 the noble metal tip is secured to the base material, 
 the main body portion of the ground electrode includes a fusion portion, the fusion portion including a constituent of the base material and a constituent of the noble metal tip, 
 a dividing cross section that is perpendicular to the opposed surface has a line that uniformly divides the opposed surface into two, the line extending the opposed surface in a cross section of the ground electrode along a longitudinal direction of the ground electrode, wherein 
 among straight lines perpendicular to a direction where the opposed surface extends and overlap a cross section of the fusion portion, a straight line at a most front end side is referred to as a first straight line and a straight line at a rearmost end side is referred to as a second straight line, 
 an area of the cross section of the fusion portion is referred to as a first area S 1 , 
 on a cross section of the main body portion of the ground electrode, an area of a part sandwiched between the first straight line and the second straight line is referred to as a second area S 2 , 
 an area ratio S 1 /S 2  is 1/3 or more, and a cross section of the core portion contacts the cross section of the fusion portion.